In recent years, electromagnetic interferences (EMI) have rapidly increased with an increase in utilization of various electric installations and electronics-applied equipments It is pointed out that EMI not only causes malfunctions and damages to electronic or electric equipments, but also damages health of operators of these apparatuses It is therefore required to suppress intensity of electromagnetic waves emitted from the electronic or electric equipments within the range of standards or regulations
As a countermeasure for the above-mentioned EMI, it is necessary to shield electromagnetic waves, and for that purpose, it is self-evident that what is necessary is just to utilize the property of metals of not transmitting the electromagnetic waves For example, there are adopted a method of using a metal body or a highly conductive body as a casing, a method of inserting a metal plate between circuit boards, a method of covering a cable with a metal foil, and the like However, an operator needs to recognize characters and the like displayed on a screen of CRT or PDP, so that a display is required to have transparency Accordingly, the above-mentioned methods have been unsuitable as a method for shielding the electromagnetic waves, because all the above-mentioned methods often provide an opaque front face of the display
In particular, PDP generates electromagnetic waves in a large amount compared to CRT or the like, so that stronger electromagnetic wave-shielding ability has been desired The electromagnetic wave-shielding ability can be conveniently represented by the surface resistance value For example, the surface resistance value is required to be about 300 Ω/sq or less for a light-transmitting electromagnetic wave-shielding material for CRT, whereas it is required to be 2 5 Ω/sq or less for a light-transmitting electromagnetic wave-shielding material for PDP In a household plasma television utilizing PDP, it is highly required to be 1 5 Ω/sq or less, and more desirably, an extremely high conductivity of 0 1 Ω/sq or less is required
Further, as for the required level of translucency, the whole visible light transmittance is required to be about 70% or more for CRT and 80% or more for PDP, and further higher transparency has been desired
In order to solve the above-mentioned problems, various materials and methods for allowing electromagnetic wave-shielding properties, conductivity and translucency to be compatible with one another by utilizing metal meshes having apertures have hitherto been proposed, as shown below
(1) Silver Paste-Printed Mesh
For example, there has been disclosed a method of printing a paste comprising a silver powder in network form to obtain a silver mesh (for example, see JP-A-2000-13088) The silver mesh obtained by this method has the problem that the line width is thick to decrease transmittance, because it is formed according to a printing method Further, the surface resistance value is high, resulting in low electromagnetic wave-shielding ability It is therefore necessary to apply a plating treatment to the resulting silver mesh, in order to increase electromagnetic wave-shielding ability
(2) Irregular Network Silver Mesh
For example, there have been disclosed an irregular minute network silver mesh and a production method thereof (for example, see JP-A-10-340629) However, this production method has the problem that only a mesh as high as 10 Ω/sq in surface resistance value (low in electromagnetic wave shielding ability) is obtained Further, there has been the problem that the haze is as high as ten-odd percents or more to cause blurring of a display image
(3) Etching-Processed Copper Mesh Utilizing Photo-Lithography
There has been disclosed a method for forming a copper mesh on a transparent substrate by subjecting a copper foil to etching processing using photolithography (for example, see JP-A-10-41682) This method has the advantage of being able to prepare a mesh having a high aperture ratio (high transmittance), which can shield even a strong electromagnetic wave emission, because it is possible to microfabricate the mesh However, there has been the problem that a production process thereof includes extremely many steps, through which the mesh must be produced
Further, there has been the problem that the accomplished mesh shows a copper foil color, not black, because of the use of the copper foil, which causes a reduction in contrast of images in display instruments Furthermore, this mesh has the problem that the width of intersectional points in a grid pattern is thicker than that of straight-line portions, because it is produced by the etching method, and improvement of this problem has been desired in association with the problem of moire
(4) Blackening of Metallic Conductive Layer
As a preferable method for blackening metallic conductive layers, for example, a surface treatment method after blackening treatment (for example JP-A-2006-191010) has been proposed However, the surfaces of conductive layers as formed by plating, vapor deposition and the like are coarse as they are, disadvantageously requiring the enlargement of the thickness of the resulting blackening layers or subsequent additional surface treatment as a treatment for preventing uneven gloss
(5) Conductive Silver Forming Method Utilizing Silver Salt
In the 1960s, there has been disclosed a method for forming a thin film pattern of metallic silver having conductivity by a silver salt diffusion transfer process in which silver is deposited on physical development nuclei (for example, see JP-B-42-23746)
However, it does not refer to the possibility that the resulting conductive metallic silver thin film can shield electromagnetic waves emitted from image display surfaces of displays such as CRT and PDP without inhibiting the image display
Actually, according to this method, a silver thin film having a surface resistance value of 10 Ω/sq to 100 KΩ/sq is obtained However, this level of conductivity is insufficient as applications for displays such as PDP Further, also in terms of high translucency, it is not sufficient Thus, translucency and conductivity can not be compatible with each other
Accordingly, even when the above-mentioned silver salt diffusion transfer process is used as it is, it has been impossible to obtain a light-transmitting electromagnetic wave-shielding material which is excellent in light transmitting properties and conductivity, and suitable for shielding electromagnetic waves from image display surfaces of electronic display equipments
The invention also relates to a production method of a printed board (printed wiring board), so that the background art thereof will be described bellow Conventional production methods are largely divided into two types a three-layer flexible board in which a copper foil is adhered onto an insulator film using an adhesive and a desired wiring pattern is formed by a subtractive method, and a two-layer flexible board in which a desired wiring pattern is formed by a subtractive method or an additive method, using a substrate having a ground metal layer directly provided on the insulator film without using the adhesive
Further, the printed boards are classified into a flexible board and a rigid board, depending on the material used in an insulating substrate The flexible board is one in which the insulating substrate is formed of a flexible material such as a polyimide resin or a polyester resin On the other hand, the rigid board is one in which the insulating substrate is formed of a material having high hardness, such as glass or an epoxy resin
Then, the three-layer flexible board whose production method is simple generally occupies the mainstream However, with a recent increase in density of electronic equipments, the wiring width in a wiring board has also come to be required to have a narrow pitch In the case of the above-mentioned three-layer flexible board, a method for forming a wiring portion by etching has been proposed (for example, JP-A-2003-309336 and the like) However, according to this method, so-called side etching in which a side face of the wiring portion is etched occurs, so that the cross-sectional shape of the wiring portion is liable to become a trapezoid widened toward the bottom Accordingly, when etching is performed until electric insulation properties between the wiring portions are secured, the wiring pitch width becomes excessively wide There has been therefore a limitation on narrowing the wiring pitch
Further, the production process is complicated and complex, which causes the problem of increased production cost, and the etching process also has a problem with regard to environmental issues for waste liquid treatment
Then, the larger this widening of the wiring portion to the bottom by the side etching is, the thicker the thickness of the copper foil becomes Accordingly, in order to decrease the widening to narrow the pitch, it has been necessary to use a copper foil having a thickness of 18 μm in place of the copper foil having a thickness of 35 μm which has hitherto been conventionally used However, such a thin copper foil has low rigidity, so that handling properties are poor Accordingly, there has been the problem that rigidity must be increased by laminating a reinforcing material such as an aluminum carrier Further, such lamination has also raised the problem of increased variations in film thickness or coating defects such as the occurrence of pinholes or cracks
Accordingly, the thinner the thickness of the copper foil is made in order to narrow the pitch of the wiring portion, the more difficult the production of the wiring board becomes, resulting in increased production cost In particular, there have recently increased demands to a wiring board having a narrow-pitch wiring portion which can not be produced without using a copper foil having a thickness of ten-odd micron meters or less, particularly several micron meters, so that the increased production cost of the three-layer flexible board has increasingly become a problem
Then, the two-layer flexible board in which a copper coating can be directly formed on the insulator film without applying the adhesive has come to be noted In the two-layer flexible board, the ground metal layer is directly formed on the insulator film by dry plating or wet plating without using the adhesive, and the copper conductive layer is formed thereon by an electroplating method It has therefore the advantages that the thickness of the substrate itself can be decreased, and moreover, that the thickness of the copper conductive coating to be adhered can also be adjusted to any thickness
A forming method of the ground metal generally employed at present in order to obtain the two-layer flexible board of this kind is a dry plating method There has been proposed a method of adhering the ground metal layer onto the insulator film, and then, further forming the copper coating thereon by dry plating (for example, JP-A-8-139448 and JP-A-10-154863) However, usually, many pinholes having a size of tens to hundreds of micron meters are present in the coating obtained by the dry plating method The final thickness of the copper coating formed is from about 0 2 to about 0 5 μm, and exposed parts caused by pinholes frequently occur in the two-layer flexible board In order to prevent this, it is necessary to repeat ground formation or to impart a catalyst Accordingly, a problem has arisen in terms of production cost